1. Field of the Invention
This invention relates to data handling systems and more particularly to apparatus for controlling the granting of service to elements of the system in response to requests for service in accordance with a predetermined priority sequence.
2. Description of Prior Art
In data handling systems information transfer occurs between system elements such as a main system and various subsystems. Some of these subsystems transfer information at a rate higher than other subsystems and, therefore, must be granted service at a rate higher than other subsystem elements. This has given rise to the development of priority circuits for ranking subsystem elements in some predetermined priority order, the higher priority being given to the faster subsystem elements. One prior circuit places the highest priority element in the first priority position in a ranking order. The requests from each element are then examined by taking each element in sequence in the order. This type of circuit has the disadvantage that higher speed subsystems are not again tested for requests until all other subsystems have been serviced.
In order to overcome this shortcoming, priority circuits have been devised which grant the highest priority subsystem service and then return to the high end of the priority ranking order. This type of circuit has the disadvantage that lower priority devices may not receive service for a long period of time since the high priority devices will effectively lock them out.